Ultrasound has been shown to have a number of clinical applications. Among these are thermal therapy, enhancement of sono-chemical reactions, and vibroacoustography.
When used for thermal therapy, ultrasonic energy is directed deep within tissue. As the tissue absorbs energy, its temperature rises by as much as 30-55 degrees C. It does so rapidly enough so that heat has no time to dissipate significantly into surrounding tissue. As a result, the ultrasound can cause necrosis and/or coagulation of the target tissue without inflicting significant harm on surrounding tissue. Because this procedure is non-invasive, no surgery is required, thus reducing the cost of therapy.
Another clinical application of ultrasound is that of enhancing sonochemical reactions for therapeutic purposes. Sonochemical reactions in liquids are known to arise from acoustic cavitation, a process that begins with nucleation, followed by growth and collapse of microscopic bubbles. The high temperatures (several thousand degrees Celsius) and high pressures (several hundred atmospheres) temporarily created in the vicinity of the bubble as it collapses are believed to trigger sonochemical reactions. Such sonochemical reactions are known to be enhanced in the process of multiple sonication frequencies.
Yet another application of clinical ultrasound is ultrasound-stimulated vibro acoustography (USVA) to create a map of the mechanical response of object to a force that is applied at each point on the object. USVA involves simultaneous output of two similar frequencies to cause spatial interference that contains, as one component thereof, a frequency-difference component having a frequency that is much lower than either of the frequencies used for sonication. In response to the force at the this difference frequency, a portion of the object vibrates. The size of this portion, and the pattern of the resulting motion, depend in part on the object's viscoelastic characteristics. The acoustic field resulting from object vibration is then detected by a hydrophone and used to form an image of the object. Because of the high spatial definition of ultrasound radiation force and high motion-detection sensitivity offered by the hydrophone, this method can identify changes in the elasticity of soft tissue, which in turn is useful for diagnosis.
A difficulty that can arise in the use of ultrasound in these and other applications is its propensity to generate standing waves when propagation occurs inside certain cavities. For example, when transcranial ultrasound is used to deliver energy into the brain, a standing wave can arise within the cranium. This standing wave can form local hot spots, or concentrations of energy.
A known way to apply ultrasonic energy in any of the above applications is to use an ultrasound phased arrays having multiple transducers. Unlike a single-element transducer, which has a fixed focus produced at a geometric center thereof, an ultrasound phased array can steer the ultrasonic energy focus to an arbitrary position. This is typically achieved by driving each transducer with a signal of the appropriate phase. Such an array can also achieve dynamic focal beam scanning by electronically altering the relative phases of the transducer signals.
However, known ultrasonic phased arrays are hampered in their ability to enhance any of the above treatment modalities by difficulties associated with simultaneously outputting multiple frequencies.